The fabrication of devices in microelectronics or in microtechnology requires, in the most advanced approaches, the production of air gaps.
To produce these air gaps, one of the current approaches consists in degrading a sacrificial material, typically silicon oxide, by means of a chemical etchant, for example hydrofluoric acid, which must pass through a membrane in order to reach this material.
An example of the production of air gaps using this approach, in the case of a structure having two levels of interconnects for integrated circuits, is illustrated schematically in FIGS. 1A and 1B.
As may be seen in FIG. 1A, which shows the structure before the air gaps are formed, said structure comprises:
a substrate 10;
a first layer 11 of a sacrificial material which covers this substrate and embedded in which are metal lines 12a and 12b, typically made of copper;
a permeable membrane 13 which covers the layer 11 and the metal lines 12a and 12b and which delimits, together with the substrate 10 and said metal lines, cavities 15 filled with sacrificial material; and
a second layer 11′ of sacrificial material, which covers the permeable membrane 13 and embedded in which are metal lines 12c and 12d, said lines being connected to the metal lines 12a and 12b respectively by means of metal vias 14 that extend right through the thickness of the membrane 13.
The contacting (shown symbolically by the bold arrows in FIG. 1A) of the structure with an agent capable of degrading the sacrificial material causes the degradation of the second layer 11′ of sacrificial material and then, after the etchant has diffused through the permeable membrane 13, that of the first layer 11 of sacrificial material. All that is then required is to remove the chemical etchant from the structure (together with all the degradation residues that it contains), for example using supercritical carbon dioxide, in order for the cavities 15 initially filled with sacrificial material to become air gaps, as illustrated in FIG. 1B.
In addition to being able to let the chemical etchant pass through it, the membrane must satisfy a very precise specification, namely:
it must itself resist the chemical etchant;
it must be compatible with the various processes and treatments used to produce the structure in which it is integrated (metallization operations, chemical-mechanical polishing operations, thermal annealing operations, and the like) and, in particular, it must be stable to temperatures that may reach 400° C.;
it must have good mechanical properties since it forms part of the framework of the structure; and
it must have a low dielectric constant, in particular at most equal to 4.0, in the case of an interconnect structure for integrated circuits.
The permeable membranes currently used are generally membranes made of polymers.
However, the integration of such membranes into microstructures, and in particular into air-gap interconnect structures for integrated circuits, cannot be accomplished without posing a number of problems.
This is because, owing to their permeability, these membranes may be infiltrated by the various chemicals that are employed during the fabrication of the microstructures and, in particular, during the operations of etching, stripping, cleaning and depositing the metals needed to produce the metal lines, thereby tending to embrittle these membranes and exposing them to a serious risk of degradation.
In addition, the fact that these polymer-type membranes are not very mechanically strong also poses problems during the implementation of abrasive operations, such as chemical-mechanical polishing operations.